The invention relates to tunable optical filters for optical communications systems, and more particularly to a tunable optical filter that utilizes electroholographic filter elements.
The desire to transmit more information over optical fibers has led to the multiplexing of multiple optical carriers at different wavelengths into the same optical fiber (wavelength division multiplexing (WDM)). Improved optical sources have enabled the generation of optical carriers with narrower bandwidths, which in turn has allowed more optical carriers to be multiplexed into the same optical fiber. As the number of multiplexed optical carriers increases, there is a need for tunable optical filters that can be adjusted to isolate specific optical carriers from a WDM signal.
Some known tunable optical filters utilize microelectromechanical systems (MEMS) or diffraction gratings to selectively filter out a specific optical carrier. These known MEMS-based and grating-based tunable optical filters utilize mechanically moving parts to tune the filters over a range of wavelengths. Utilizing mechanically moving parts to tune filters makes the filters susceptible to vibration and mechanical failure.
An alternative technique for filtering an optical signal utilizes a volume hologram written into a photorefractive crystal to create a narrow-band optical filter. Utilizing a volume hologram written into a photorefractive crystal to create a narrow-band optical filter is described by George A. Rakuljic and Victor Leyva in xe2x80x9cVolume Holographic narrow-band optical filterxe2x80x9d, Optics Letters, vol. 18, No. 6 pp. 459-461 (1993). Although writing volume holograms into photorefractive crystals works well to create a filter with a fixed narrow band, a single fixed narrow-band filter does not fulfill the need for wide-band tunable optical filters for use in WDM-based optical communications systems.
Recently, voltage controlled volume holograms written into photorefractive crystals have been incorporated into optical communications devices such as optical switches. Optical switches that incorporate voltage controlled volume holograms written into photorefractive crystals are described in detail in international patent applications published under the Patent Cooperation Treaty (PCT) entitled xe2x80x9cElectro-Holographic Optical Switchxe2x80x9d (WO 00/02098) and xe2x80x9cElectroholographic Wavelength Selective Photonic Switch For WDM Routingxe2x80x9d (WO 01/07946).
In general, the principle of operation of photorefractive crystals involves writing a grating pattern into the crystal by establishing a periodic space-charge field. In the paraelectric region, the electro-optic effect is quadratic and is given by:
xcex94n=xc2xdno3gP2xe2x80x83xe2x80x83(1)
where xcex94n is the birefringence, no is the refractive index, g is the appropriate electro-optic coefficient, and P is the static polarization. In the linear region, P is given by P=∈o (∈xe2x88x921)E, where ∈ is the dielectric constant (which when close to the phase transition follows ∈/∈o greater than  greater than 1) and where ∈o is the permittivity of a vacuum 8.854xc3x9710xe2x88x9212 F/m. When the interference patterns are written into the crystal using two optical beams, the space-charge fields, Esc, that are created are spatially correlated to these patterns. These space-charge fields induce refractive index gratings in the presence of an external electric field, Eo, that is given by:
xcex94n=xc2xdno3g∈o2(∈xe2x88x921)2(2EoEsc+Esc2)xe2x80x83xe2x80x83(2)
where Eo is the externally applied field. It is assumed that the polarization is in the linear region.
The mechanism described with reference to equation (2) provides the ability to selectively activate or de-activate the filtering capability of a photorefractive crystal based on the presence or absence of an external electric field. FIG. 1 depicts a photorefractive crystal 102 with an electroholographic (EH) grating 104 that is activated through a voltage source. When the voltage source is in the xe2x80x9coffxe2x80x9d state (no voltage applied), the optical signal passes, undiffracted, through the electroholographic grating as indicated by dashed line 106. However, when the voltage source is in the xe2x80x9conxe2x80x9d state (voltage applied), the optical signal at the center wavelength of the gratings is diffracted by the grating as indicated by the solid line 108. Because the grating is wavelength specific, only optical signals within the bandwidth of the grating are diffracted.
It is well known in the field of optics that for incident light of wavelength xcex, the response of a grating is given by:
d sin (xcex8)=mxcexo/2nxe2x80x83xe2x80x83(3)
where, d is the spacing between the lines of a grating and xcex8 is the incident angle of light, m is an integer, n is the index of refraction of the crystal and xcexo is the wavelength of the incident light. For a grating with fixed d and xcex8, any change in the index of refraction results in a variation of the center wavelength of the grating. As described above, the index of refraction of a photorefractive crystal and in turn the center wavelength of the EH grating changes as a function of the externally applied electrical field. For example, a grating written into a material such as strontium barium niobate (SBN):75 can experience as much as 0.5% variation in its index of refraction in the presence of a 330 volts/cm external electric field. For operation around 1,500 nm, a 0.5% variation in the index of refraction allows a grating to be tuned over a range of about 7.5 nm. Although a single electroholographic grating written into a photorefractive crystal can be tuned over a range of about 7.5 nm, WDM communications systems operate over a larger bandwidth, for example, 100 nm and therefore a single filter is not well suited for WDM applications.
In view of the need for tunable optical filters and the problems with mechanically tuned filters, there is a need for a robust tunable optical filter with a tuning range that is compatible with WDM communications systems.
Multiple electroholographic (EH) gratings with different center wavelengths are utilized to create a tunable optical filter that can be tuned over a wide wavelength range. The EH gratings are connected such that an input optical signal can pass through at least one of the EH gratings. The EH gratings are activated and tuned by electrode pairs that are controlled through a voltage controller. The tunable optical filter is coarse tuned by activating the EH gratings having a wavelength range that includes the center wavelength that is to be filtered and fine tuned by adjusting the voltage that is applied across the activated EH gratings. Because the tunable optical filter is tuned simply by the application and adjustment of voltage across EH gratings, the tunable optical filter can be accurately controlled and is less susceptible to vibration and mechanical failure. In addition, because the filter utilizes multiple EH gratings with different center wavelengths, the bandwidth of the filter can be extended beyond the bandwidth of any single EH grating.
An embodiment of a tunable optical filter includes multiple EH gratings with different center wavelengths. The EH gratings are optically connected such that an input optical signal can pass through at least one of the EH gratings. The EH gratings are activated to filter the input optical signal in response to an applied voltage.
In an embodiment, the EH gratings of the tunable optical filter are activated by electrode pairs that are associated with the EH gratings and the electrode pairs are controlled by a voltage controller. In an embodiment, EH gratings of the same center wavelength are controlled simultaneously by the voltage controller.
The tunable optical filter can be tuned over a range of wavelengths in response to adjustments in the applied voltage. In an embodiment, the tunable wavelength ranges of the EH gratings combine to form a continuous wavelength range that is greater than the wavelength range of any one EH grating.
Although the EH gratings are optically connected such that an input optical signal can pass through at least one of the EH gratings, typically, the EH gratings are optically connected such that an input optical signal can pass through at least two EH gratings having different center wavelengths in series. In an embodiment, a birefringent element can be used to split the input optical signal into two polarized beams to ensure polarization diverse filtering. When the input optical signal is split into two polarized beams, the EH gratings may include a first group of EH gratings having different center wavelengths that are optically connected such that the first polarized beam can pass through the first group of EH gratings and a second group of EH gratings having the same center wavelengths as the first group that are optically connected such that the second polarized beam can pass through the second group of EH gratings, with the first and second polarized beams passing through the respective groups of EH gratings in parallel.
In an embodiment, the EH gratings are formed in photorefractive crystals.
In another embodiment, the EH gratings of the tunable optical filter are included within a chirped grating.
Additional components of a tunable optical filter may include an input birefringent element, an input polarization rotator, an output birefringent element, and an output polarization rotator. The input birefringent element is located in an optical path that is before the EH gratings. The input birefringent element splits the input optical signal into first and second polarized beams having different polarization states before the input optical signal passes through the EH gratings. The input polarization rotator is located in an optical path that is between the input birefringent element and the EH gratings. The input polarization rotator brings the first and second polarized beams to the same polarization state. The output birefringent element is located in an optical path that is after the EH gratings. The output birefringent element combines the first and second polarized beams into an output signal after the first and second polarized beams have passed through the EH gratings. The output polarization rotator is located in an optical path that is between the EH gratings and the output birefringent element. The output polarization rotator brings the first and second polarized beams to different polarization states. In one arrangement of the filter, a first set of electroholographic filter elements (EFEs), which includes a first group of the EH gratings, are aligned to filter the first polarized beam and a second set of EFEs, which includes a second group of the EH gratings, are aligned to filter the second polarized beam.
Because some activated EH gratings cause the polarization state of a diffracted optical signal to be rotated, an embodiment of the tunable optical filter includes polarization rotators that are located between EH gratings of the same center wavelength to counteract the rotation that is caused by the activated EH gratings.
An embodiment of a method for filtering an optical signal that utilizes EH gratings involves passing an optical signal through a series of EH gratings with different center wavelengths, with the EH gratings being activated in response to an applied voltage, and selectively applying a voltage across at least one of the EH gratings to activate the at least one EH grating, thereby filtering the optical signal at a desired center wavelength. The filter can be fine tuned by adjusting the voltage that is applied across an activated EH grating. The filter can be coarse tuned by applying a voltage to a different one of the EH gratings. The filter can be tuned across a range of wavelengths by serially activating and tuning different sets of the EH gratings.
In an embodiment of the method, voltage is simultaneously applied across a set of EH gratings that have the same center wavelength.
In an embodiment of the method, the optical signal is split into two polarized beams before the optical signal is passed through the series of EH gratings and the polarization state of one of the beams is rotated such that the two polarized beams have the same polarization state before the two polarized beams are passed through the series of EH gratings. The two polarized beams are recombined after the beams have passed through the series of EH gratings.
Another embodiment of a tunable optical filter includes multiple EFEs that are optically aligned in a series of sets, such that an input optical signal can pass through each set of EFEs. Each set of EFEs includes EH gratings that have different wavelength ranges than the other sets of EFEs. Electrode pairs associated with each of the EFEs are used to activate the EH gratings within the EFEs and a voltage controller associated with the electrode pairs controls the application of voltage to the EFEs. The EH gratings within the EFEs can be tuned over their respective wavelength ranges by adjusting the applied voltage. A continuously tunable optical filter is formed by combining EFEs with overlapping wavelength ranges.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.